1. Field
One embodiment of the invention relates to an electronic device and, more particularly, to power saving control of the electronic device.
2. Description of the Related Art
These days, the heat generation amount of a CPU incorporated in an electronic device such as a notebook computer becomes larger. In order to diffuse heat from the CPU, heat is generally transferred to a radiating portion (heat sink, fan, or the like) via a heat pipe and diffused from the radiating portion. A liquid coolant is filled in the heat pipe. At one end of the heat pipe that is connected to a heat receiving portion, the internal liquid coolant evaporates by heat transferred from the CPU. The evaporated gas deprives the CPU of heat, and transfers heat to the radiating portion. The radiating portion condenses gas into a liquid coolant by thermal diffusion and cooling by a radiation fin, fan, or the like. The condensed liquid coolant returns to the end of the heat pipe on the side of the heat receiving portion by the action of, e.g., a wick (thin groove for moving the liquid coolant by capillarity) in the heat pipe, and the heat transfer process is repeated. In a normal device use state, the end of the heat pipe on the side of the radiating portion is desirably prevented from becoming much lower than an end on the side of the heat receiving portion in order to move the condensed liquid coolant from the radiating portion to the heat receiving portion. This structure can smoothly move the liquid coolant from the radiating portion to the heat receiving portion (see, e.g., Jpn. Pat. Appln. KOKAI Publication No. 11-17375 (FIGS. 1, 4, 6, and 7)).
The portability of most recent electronic devices has been improved, and they can generally operate even while they are carried. Depending on how the user carries the electronic device, the electronic device is not always in the above-described ideal use state in which the radiating-portion side of the heat pipe is higher than its heat receiving-portion side. For example, the electronic device may operate while the radiating-portion side of the heat pipe is lower than its heat receiving-portion side (which will be referred to as a top heat state hereinafter). In this case, the liquid coolant moves from the radiating portion to the heat receiving portion against gravity, and moves through the wick in the heat pipe against gravity. Therefore, the heat receiving-portion side of the heat pipe does not keep a sufficient amount of liquid coolant, and the original heat transfer ability degrades. If the amount of heat generated by the CPU is large in this state, all the liquid coolant on the heat receiving-portion side of the heat pipe evaporates, and the original heat transfer ability of the heat pipe is completely lost (this state is also called top heat). As a result, the electronic device must be completely turned off.
It is disclosed by, for example, Jpn. Pat. Appln. KOKAI Publication No. 11-17375 that a technique using a heat pipe which extends crosswise from a heat receiving portion. According to this technique, even when the device is inclined, a liquid coolant returns from one heat pipe portion of the cross-shaped heat pipe, and the heat transfer ability is maintained to a certain degree. However, the heat pipe disclosed in Jpn. Pat. Appln. KOKAI Publication No. 11-17375 requires a large installation area in the device, which is disadvantageous for cost, assembling, and the degree of freedom of design.